1. Field of the Invention
The present invention relates to a real image type finder optical system for use in a viewfinder of cameras. The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei 10-344339 (filed on Dec. 3, 1998), which is expressly incorporated herein by reference in its entirety.
2. Description of the Related Art
As shown in the plan view of FIG. 12, in a conventional real image type finder optical system for use in compact cameras and the like, light beams from an object are transmitted through an objective optical system 100 to be converged, reflected 90.degree. laterally by a roof mirror 101 while being inverted upside down along the vertical direction that is perpendicular to the plane of FIG. 12, and forms an erected image of an object on the plane of a condenser lens 102 once. The light beams having formed the erected image of the object enter a penta prism 103 while being diverged. The light beams are reflected twice, each by 45.degree., within the plane including optical axes of the objective optical system 100 and the condenser lens 102 in the pentagonal prism 103 and thereafter exit the penta prism 103 in parallel to the optical axis of the objective optical system 100. The light beams coming out of the penta prism 103 are transmitted through an eyepiece lens 104 to form the virtual image of the object which is observed by a photographer. Incidentally, the cover glass 105 in FIG. 12 is a plane parallel plate fit into the camera casing.
Since its optical axis extending from the objective lens 100 to the eyepiece lens 104 (hereinafter, referred to as "finder optical axis") is bent only within the same plane, a conventional real image type finder optical system having such configuration as described above may be small in vertical dimension as compared with ones using a Porro prism.
Now, for the purpose of making a diopter adjustment in conformity to the photographer's vision possible, it is desirable to design the eyepiece lens 104 so as to be movable along the direction of the finder optical axis. Therefore, a moving range .alpha. of the eyepiece lens 104 must be provided behind the exit surface 103a of the penta prism 103.
According to the conventional real image type finder optical system shown in FIG. 12, however, the entire finder optical path extending from the objective optical system 100 to the fore-end of the moving range .alpha. of the eyepiece lens 104 (that is, the exit surface 103a of the penta prism 103) lies before (above, in FIG. 12) the fore-end of the moving range .alpha. of the eyepiece lens 104 (that is, the exit surface 103a of the penta prism 103). This means that the moving range .alpha. of the eyepiece lens 104 can not overlap with the finder optical path extending from the objective optical system 100 to the fore-end of the moving range .alpha. of the eyepiece lens 104 (that is, the exit surface 103a of the penta prism 103) in the front-to-back direction (the top-to-bottom direction in FIG. 12). Consequently, conventional real image type finder optical systems cannot be constituted very compact in overall length along the front-to-back direction, and the cover glass 105 needs to be projected from the camera casing.